Process for sweetening and stabilizing cracked gasolines



May 20, 1958 c. s. COE 2,835,564

PROCESS FOR SWEETENING AND STABILIZING CRACKED GASOLINES Filed June 25.1956 [av-law Jill/ii wry 7a PROCESS FOR SWEETENING AND STABILIZINGCRACKED GASOLINES Clarence S. Coe, Rolling Hills, Calif., assignor toUnion Oil Company of California, Los Angeles, Qalifi, a corporation ofCalifornia Application June 25, 1956, Serial No. 593,631

2 Claims. (or. 44-78) This invention relates to methods for sweeteningcracked gasolines, i. e. removing mercaptans, while also preventingoxidative changes normally resulting in gum formation. The sweeteningmethods employed herein comprise the aqueous caustic, or causticmethanol-water, extraction procedures which are per se well-known in theart. These caustic sweetening methods however do not efiectstabilization of such gasolines, and will sometimes accelerate gumformation. According to the present invention, a novel inhibitor isadded to the gasoline prior to caustic washing, whereby peroxidation maybe prevented before, during, and following the caustic scrubbingoperation, to a greater degree than has been heretofore possible. I

The use of phenolic inhibitors for preventing oxidation of crackedgasolines is well-known in the art. However, the majority of suchphenolic compounds are soluble in caustic solutions, and especially incaustic methanolwater. Hence, such phenolic inhibitors can only be addedto the gasoline following the caustic washing treatment. This isdisadvantageous because it is frequently desirable to store the gasolinefor considerable periods of time prior to the caustic washing treatment.More'- over, in the caustic treatment itself, air is ordinarily notexcluded from the scrubbing tower, and the presence of air and caustictends to accelerate the formation of peroxides and gums in the gasoline.It would hence be highly desirable to add the inhibitor to the gasolineimmediately following the run-down of gasoline from the distillationcolumn, and in any event prior to the caustic washing treatment.

One of the most widely used phenolic inhibitors. is2',6-di-tert-butyl-p-cresol. This material is an. active stabilizer, andpossesses a fair degree of insolubility in caustic solutions. even moreeffective, and caustic insoluble, phenolic stabilizer. This materialconsists of the novel compound 3 ,3',5 ,5 -tetratert-butyl-4.4'dihydroxy diphenyl, having the structural formula:

It is accordingly the principal object of this invention to providemethods for effecting caustic washing of cracked gasolines in thepresence of a highly effective oxidation inhibitor. A further object isto provide a phenolic oxidation inhibitor which isinsoluble in aqueouscaustic, and in caustic methanol-water, and which may therefore be addedto the gasoline prior to caustic treatment. Another object is to providea phenolic inhibitor of higher potency than has heretofore beenavailable. Other objects will appear from the following description.

U. S. Patent No. 2,479,948 discloses a class of inhibitors which may bedesignated as 3,3,5,5'-tetra-alkyl-4,4- dihydroxy diphenyls, wherein notmore than two of the However; I have now discovered an r alkyl groupsare tertiary alkyl groups. The compound3,3',5,5'-tetra-tert-amyl-4,4'-dihydroxy diphenyl is disclosed therein,but is shown to be ineffective as an inhibitor, being less active thanordinary cresylic acids. The other compounds disclosed, which containprimary and/or secondary alkyl groups, while stated to be relativelyactive as inhibitors, are appreciably soluble in alkaline solutions. Thepresent invention embodies mydiscovery' that the specific compound3,3',5,S-tetra-tertbutyl-4,4'-dihydroxy diphenyl is highly active as aninhibitor and is also very insoluble in alkaline solutions. This novelcompound is in fact more active than any other known dihydroxy diphenylinhibitor, and is also more active than di-tert-butyl p-cresol.

The accompanying drawing illustrates schematically one method ofpracticing the invention, but is not intended to be limiting in scope.

The cracked gasoline from the initial distillation is brought in throughline 1, and admixed in mixing valve 2 with inhibitor added via line 3from storage tank 4. The

(inhibitor may be added in the solid state, or it may be added as aconcentrated solution in benzene or other hydrocarbon, acetone, or otheroil-miscible compound. The proportion of inhibitor to be added shouldrange between about 0.00l%-0.2% by weight, although any minor proportionis effective in some degree. The gasoline containing the inhibitor isthen transferred via line 5 to run-down storage tank 6, where it may bestored for any desired length of time, prior to caustic washing.

As the stabilized gasoline is desired for caustic treatment, it iswithdrawn through line 8 and transferred to the bottom of causticscrubbing column 9, which may be any conventional type for effectingcountercurrent liquidliquid extraction, e. g. a column packed with glassbeads, porcelain chips or the like. In column 9 the gasoline flowsupwardly, andcont'acts descending caustic methanolwater, which efifectsextraction of substantially all of the mercaptans, but does not extractany significant proportion of the inhibitor. In the preferred operation,recycle methanol is added at a mid-point in the column via line 10,while aqueous caustic is admitted at a higher point in the column vialine 11. In this manner any dissolved or entrained methanol in thegasoline is removed between the inlet points of lines'10 and 11. Theresulting gasoline withdrawn through line 13 is substantially free ofmercaptans and methanol, but may contain traces of dispersed. causticsolution, and hence may be scrubbed in column. 14, countercurrently towater ad mitted through line 15. Spent wash water is Withdrawn throughline 16. The final gasoline is withdrawn through line 17 and sent tostorage indicated at 18. The gasoline in storage tank 18. still containssubstantially the same amount of inhibitor as was added at mixing valve2, and

may hence be stored indefinitely while obtaining the full.

elfects' of the inhibitor.

The spent mixture of methanol, water, caustic and dissolved mercaptidesis withdrawn from column 9 through line 20 and sent for regeneration toa stripping column 21. In this. column, which is maintained at a bottomstemperature between about and 250 C., the mercaptides are substantiallycompletely hydrolyzed,

and a mixture of free mercaptans, methanol, and water is distilledoverhead. This mixture is then condensed incooler. 23-andlsenttoadecanter 24 via line 22. Liquid.

mercaptans form a supernatant phase which is removed via line 27, andmay be subsequently treated for the recovery of the mercaptans bymethods forming no part of the present invention.

The lower layer of methanol and .water in decanter 24 is withdrawn vialine 25 and sent to a methanol recovery column 26 wherein methanol isdistilled overhead and recycled via line 10 to a mid-point of causticscrub- 3 bing column 9, as previously described. The aqueous bottomsfrom column 26 is withdrawn through line 29 and may be discarded ifdesired. In order to make up for the loss of water in column 26, it isusually desirable to inject steam into stripping column 21 via line 30.

The aqueous caustic which is recycled through line l l preferablyconsists of sodium hydroxide containing between about 20-40% by weightof NaOH. If potassium hydroxide is employed a somewhat higher proportionis preferable, e. g. 25-60 weight-percent. The methanol recycled vialine may be substantially anhydrous, or the distillation in column 26may be operated to obtain a rougher separation, whereby some waterreturns via line 10. The volume of methanol recycled via line 10 shouldpreferably correspond to about 10-40 volumes per 100 volumes of aqueouscaustic recycled via line 11. Caustic extraction column 9 is ordinarilyoperated at substantially atmospheric temperatures, but any temperaturebetween about 50l50 F. may be employed. Under these conditions it isfound that substantially none of the inhibitor is removed from thegasoline stream passing through extraction column 9.

While the above illustration shows the use of methanolcontainingcaustic, other caustic treating solutions may be employed. In thetreatment of light naphthas, especially those containing only smallamounts of mercaptan sulfur, the methanol may be unnecessary. Lowermercaptans are sufliciently soluble in aqueous caustic alone. Howeverfor stocks containing appreciable quantities of higher mercaptans, theuse of a solutizer such as methanol is usually desirable. Othersolutizers iriflude ethanol, isopropanol, cresols, acetone and the li e.

The compound 3,3',5,5'-tetramethyl-4,4'-dihydroxy diphenyl issubstantially soluble in 10% aqueous sodium hydroxide, and is hence notusable in the present invention. The compounds,3,3-dimethyl-5,5'-di(tert-butyl)- 4,4-dihydroxy diphenyl, and3,3',5,5'-tetra-isopropyl- 4,4-dihydroxy diphenyl, are slightly solublein 10% sodium hydroxide, and are substantially soluble in cansticalcohol, and are hence not usable herein. The compound,3,3,5,5'-tetra(tert-amyl)-4,4'-dihydroxy diphenyl is insoluble in allcaustic solutions, but is only about one-tenth as active as thetetra-tert-butyl compound. Only the tetra-tert-butyl compound appears topossess the desired combination of caustic insolubility and activitywhich is essential herein.

The novel inhibitor of this invention may be prepared by the methodsdescribed in U. S. Patent No. 2,479,948, or preferably by the methoddescribed in my copending application Serial No. 269,891, filed February4, 1952, now U. S. Patent No. 2,785,188. In the latter method, a2,6-di-tert-butyl-4-halophenol is treated with a mild oxidizing agentsuch as air, in the presence of a copper catalyst and an alkalinematerial, whereby there is formed3,3'-5,5'-tetra-tert-butyl-4,4-diphenoquinone. The diphenoquinone isthen subjected to mild hydrogenation, as for example by means of zincdust and acetic acid, to form the diphenol.

The potency of gasoline stabilizers may be evaluated by means ofperoxide number determinations. In this procedure, the gasoline sampleis exposed to air for varying lengths of time, and is then titrated witha reducing solution consisting essentially of 0.01 N-titanous chloride,in order to determine the peroxide content, and thus the amount ofoxygen taken up. This method is more particularly described in U. 0. P.Laboratory Test .4 Methods for Petroleum and its Products, Universal OilCompany (1940).

Example In order to evaluate the stabilizing activity of the inhibitorof this invention, peroxide numbers were determined for three samples,A, B, and C, of a light cracked gasoline boiling between 100300 F., andderived from a California crude oil. Sample A contained no inhibitor;sample B contained 0.0035% of 2,6-di-tertbutyl p-cresol; and sample Ccontained 0.0035% of 3,3-

dihydroxy diphenyl is effective in extremely small amounts forpreventing the formation of peroxides in cracked gasolines, and that itretains its activity for a longer period of time than does2,6-di-tert-butyl p-cresol. Inasmuch as these peroxides are consideredto be precursors of, or essential factors in, the formation of gums ingasolines, it is clear that the above compound is a highly effectivestabilizer.

When the procedure employed for testing sample C is repeated, following10 minutes of shaking the gasoline sample with a caustic solutioncomposed of 100 volumes of 20% aqueous NaOH solution to which 20 volumesof methanol has been added, the peroxide numbers of the caustic-washedgasoline after 24 hours, 72 hours and 17 days exposure to air aresubstantially the same, respectively, as indicated above. No detectableamounts of inhibitor are present in the caustic solution.

The foregoing disclosure of this invention is not to be considered aslimiting since many variations may be made by those skilled in the artwithout departing from the scope or spirit of the following claims..

I claim:

1. A method for stabilizing and sweetening a cracked gasoline whichcomprises first dissolving in said gaso line a minor proportion, lessthan about 0.2%, of 3,3',5,5',tetra(tert butyl) 4,4 dihydroxy diphenyl,then subjecting said gasoline to caustic extraction with a causticsolution to extract mercaptans without extracting substantialproportions of said inhibitor, said caustic solution consisting of analkali metal hydroxide dissolved in a solvent which is predominantlywater.

2. A process as defined in claim 1 wherein said caustic solution is acaustic methanol-water solution containing between about 10 and 40volumes of methanol dissolved in 100 volumes of an aqueous causticsolution,

I said aqueous caustic solution containing between about 20% and 40% byweight of NaOH.

Luten et al Aug. 23, 1949 Rosenwald Nov. 4, 1952

1. A METHOD FOR STABILIZING AND SWEETENING A CRACKED GASOLINE WHICHCOMPRISES FIRST DISSOLVING IN SAID GASOLINE A MINOR PROPORTION LESS THANABOUT 0.2%, OF 3,3''5,5'',TETRA(TERT - BUTY) -4,4'' - DIHYDROXYDIPHENYL, THEN SUBJECTING SAID GASOLINE IN CAUSTIC EXTRACTION WITH ACAUSTIC SOLUTION TO EXTRACT MERCAPTANS WITHOUT EXTRACTING SUBSTANTIALPORPORTIONS OF SAID INHIBITOR, SAID CAUSTIC SOLUTION CONSISTING OF ANALKALI METAL HYDROXIDE DISSOLVED IN A SOLVENT WHICH IS PREDOMINANTLYWATER.